The present invention relates generally to orthopaedic surgery and in particular to shoulder prostheses. More particularly, the present invention relates to an implant forming part of a humeral prosthesis. Specifically, the implant comprises an articulation component to articulate against a glenoid implant or a natural glenoid cavity. The present invention also relates to an assembled prosthesis and a shoulder arthroplasty procedure using the humeral prosthesis.
A natural shoulder joint may undergo degenerative changes for a variety of reasons, for instance arthritis. Alternatively, a shoulder joint may be fractured or otherwise damaged by an external force. When a shoulder joint is sufficiently degenerated or damaged it may be necessary to replace a natural shoulder joint with a prosthetic shoulder joint. Conventional shoulder prostheses comprise a humeral implant, optionally with a glenoid implant.
Typically, the humeral implant comprises a stem for insertion into a bore extending into the intramedullary canal generally along the longitudinal axis of the bone and an articulation component such as a convex bearing head. Typically, the articulation component is coupled to a neck portion of the stem which extends from the intramedullary canal at an inclined angle relative to the longitudinal axis of a distal portion of the stem in order to recreate the arrangement of the natural joint.
When the upper extremity of the humerus is fractured, the humerus generally breaks into several pieces, typically three or four. In particular, for a four part fracture as illustrated in FIG. 1, the humeral head 2 splits off at the level of the anatomical neck indicated by line 4, the greater tuberosity 6 and lesser tuberosity 8 are separated from the humeral shaft 10 below the tuberosities and the tuberosities 6, 8 are separated from one another along the bicipetal groove 12. In a three part fracture the greater or less tuberosity 6, 8 remains attached to the humeral head 2. For both three part and four part fractures, as there is no longer a blood supply to the humeral head 2 necrosis may begin.
For repair of a four part fracture or a three part fracture, the humeral head is replaced, and the greater and lesser tuberosities are reattached to the humeral shaft. Known shoulder prostheses for repair of a four part fracture provide tabs or fins attached to the stem which incorporate holes to receive sutures to couple the tuberosities and soft tissue such as cuff muscles to the prosthesis. Correct positioning of the suture holes is essential to accommodate the differing attachment points of soft tissues to the humerus in a natural joint. Typically three and four part humeral fractures are repaired through a hemi-arthroplasty procedure as there may be no damage to the glenoid cavity and so no need for its replacement.
Certain known prostheses suffer from inflexibility in the position of attachment points for coupling bone fragments and soft tissues to the implant, which may result in loss of mobility for the repaired joint.
UK patent application 1007656.0 filed on 7 May 2010 in the name of DePuy (Ireland) (and incorporated herein for all purposes) discloses a rotatable collar arranged to couple to a neck portion of a humeral implant, such as a humeral stem which extends into the intramedullary canal. The collar is arranged to be located between an articulation component and the natural bone surrounding the humeral implant. The collar comprises a plate defining a plurality of suture holes or pre-attached sutures around its periphery which allow the detached bone fragments, including the greater and lesser tuberosities to be coupled to the implant. A suture cage may be formed extending from the collar around the detached bone fragments and back to an opposite portion of the plate to hold the bone fragments in position. The collar causes the articulation component to be spaced apart from the bone, and may therefore interfere with the correct positioning of the articulation component.